Semiconducting conjugated polymer thin-film transistors (TFTs) or more generally field-effect transistors (FETs) have recently become of interest for applications in cheap, logic circuits integrated on plastic substrates (C. Drury, et al., APL 73, 108 (1998)) and optoelectronic integrated devices and pixel transistor switches in high-resolution active-matrix displays (H. Sirringhaus, et al., Science 280, 1741 (1998), A. Dodabalapur, et al. Appl. Phys. Lett. 73, 142 (1998)). In test device configurations with a polymer semiconductor and inorganic metal electrodes and gate dielectric layers high-performance TFTs have been demonstrated. Charge carrier mobilities up to 0.1 cm2/Vs and ON-OFF current ratios of 106-108 have been reached, which is comparable to the performance of amorphous silicon TFTs (H. Sirringhaus, et al., Advances in Solid State Physics 39, 101 (1999)).
One of the advantages of polymer semiconductors is that they lend themselves to simple and low-cost solution processing. However, fabrication of all-polymer TFT devices and integrated circuits requires the ability to form lateral patterns of polymer conductors, semiconductors and insulators. Various patterning technologies such as photolithography (WO 99/10939 A2), screen printing (Z. Bao, et al., Chem. Mat. 9, 1299 (1997)), soft lithographic stamping (J. A. Rogers, Appl. Phys. Lett. 75, 1010 (1999)) and micromoulding (J. A. Rogers, Appl. Phys. Lett. 72, 2716 (1998)), as well as direct ink-jet printing (H. Sirringhaus, et al., UK 0009911.9) have been demonstrated.
Many direct printing techniques are unable to provide the patterning resolution that is required to define the source and drain electrodes of a TFT. In order to obtain adequate drive current and switching speed channel lengths of less than 10 μm are required. In the case of inkjet printing this resolution problem has been overcome by printing onto a prepatterned substrate containing regions of different surface free energy (H. Sirringhaus et al., UK 0009915.0).
In WO0229912 and UK 0229191.2 methods are described for the fabrication of organic TFTs by embossing. The methods are based on forming a microgroove by embossing a rigid master into a substrate, and forming a field-effect device inside the embossed microgroove. The substrate can be a flexible deformable plastic substrate, a rigid substrate containing a flexible overlayer, or even a rigid substrate in the case of melt embossing. Planar-channel and vertical-channel field-effect devices with short, submicrometer channel lengths can be fabricated in this way. The embossing step is an integral part of the device manufacturing process. Microcutting is used to define the critical-feature channel length between source- and drain-electrodes of planar-channel and vertical-channel devices. The topographic profile of the embossed grooves is used to pattern the surface energy of the substrate in order to confine the deposition of an ink solution to a defined area on the substrate. An example is a self-aligned gate electrode, where the conducting ink for deposition of the conductive gate electrode is confined to the embossed grooves with the help of selective surface modification that makes the flat regions of the substrate repulsive for the deposition of the ink.
One important application of organic FETs are in sensors, such as, but not limited to, sensors of chemical, biological, or gaseous species, or temperature or humidity sensors. The sensing ability of the FET is based on some change in the electrical characteristics of the FET when exposed to an environment that contains a small concentration of the species to be detected (or upon temperature/humidity change). A range of different FET device configurations can be used for this purpose. In a chemical FET (CHEMFET) the gate electrode is formed from a material that interacts with the species to be detected, the interaction affecting the gate voltage. An example of a conventional, inorganic CHEMFET is a silicon MOSFET with a Pd gate for the detection of H2 hydrogen gas. At elevated temperatures, the hydrogen is thought to dissolve into the Pd gate, diffusing to the Pd/SiO2 interface and forming an electrical layer that affects the flat band voltage of the metal-oxide-semiconductor (MOS) structure, and shifts the threshold voltage of the FET. For given applied source, drain, and gate voltages this shift of threshold voltage results in a change of the source-drain current.
Some of the key requirements for a good FET sensor are:                Selectivity: The sensor should ideally react only to the species or environmental factor that it is meant to detect, and not be equally sensitive to the presence of other species. This requirement can be difficult to achieve, but can, for example, be relaxed by using an array of different sensors each with different sensitivities to the various different species that the sensor array can potentially be exposed to during operation.        Sensitivity: The sensor should be highly sensitive to the presence of the species to be detected. The sensitivity is dependent on both the nature of the interaction between the species and the sensor, as well as on the geometry of the sensor determining the area of interaction. In the case of many FET sensors such as a CHEMFET the sensitivity of the sensor is determined by the transconductance of the FET, i.e. the change in source-drain current in response to a change in gate voltage.        Information processing: The signal from the sensing element needs to be in such a form that it can be easily processed into a form that can be transmitted to the information gathering unit. In the case of an FET sensor this means that the current or voltage signal supplied by the FET upon detection of the species is sufficiently large to allow signal processing.        Linearty: The response of the sensor to the species to be detected should be approximately linearly dependent on the concentration of the species, in order to facilitate the calibration of the sensor.        Stability: The response of the sensor should be stable during its operation, i.e. it should always give the same response when exposed to the same concentration of species. If the response of the sensor changes in time, frequent recalibration is required which can be difficult, in particular in situations where the sensor is not easily accessible.        
In many sensor applications the sensor needs to be exposed to a stream of liquid or gas containing the species to be detected. The concentration of the species can be small, requiring a large area of interaction exposed to the flow of liquid gas. In particular, for biological sensors the volumes of liquid that contain, for example, an enzyme to be detected can be small, and sensor configuration need to be used in which the probability that the species comes in contact with the sensor, as the flow is passing by, is maximised.
Controlling the flow of liquids by using microfabricated channels (“microfluidics”) is a field with a growing number of applications. A common technique is to form a network of microfluidic channels in an elastomer sample, such as poly(dimethylsiloxane) (PDMS). The PDMS is poured over a master containing a topographic profile, that can be fabricated by techniques, such as photolithography, curing the elastomer, and removing it from the master. The PDMS sample then contains an array of recessed channels. These channels can be sealed by bonding the PDMS to a flat substrate, such as a glass substrate or another flat PDMS sample. Often an oxygen plasma treatment is used to improve the adhesion between the two substrates, which is important to seal the channels against leakage of liquid. Examples of applications of such microfluidic channels are the patterned delivery and deposition of biomolecules on surfaces (E. Delamarche et al., Science 276, 779 (1997)), the localized reaction between two species inside a microfluidic channel followed by deposition on the substrate (P. Kenis et al., Science 285, 83 (1999)), the realization of a large number of reaction chambers the contents of which can be individually controlled (T. Thorsen, et al., Science 298, 580 (2002)). In principle, a microscopic version of a laboratory can be created in this way (lab-on-a-chip). Key components of such microfluidic system such as valves and pumps have been developed (M. Unger, Science 288, 113 (2000)). In all of these applications it would be highly desirable if a diagnostic tool in the form of a sensor could be integrated into the microfluidic channels in order to measure for example the concentration of reagents, or detect the presence of a reaction product.